PUT /_settings causes NullPointerException if a system index is present

Versions affected: 7.12.0, 8.0.0

Steps to reproduce:
```sh
# Perform a reindex so that the .tasks index is created
$ curl -X PUT "elastic:changeme@localhost:9220/my-index?pretty" -H 'Content-Type: application/json'
{
  "acknowledged" : true,
  "shards_acknowledged" : true,
  "index" : "my-index"
}
$ curl -X POST "elastic:changeme@localhost:9220/_reindex?wait_for_completion=false" -H 'Content-Type: application/json' -d '{"source": {"index": "my-index"}, "dest": {"index": "my-index-2"}}'

# Update all indices' settings
$ curl -X PUT "elastic:changeme@localhost:9220/_settings?pretty" -H 'Content-Type: application/json' -d'
{
    "index": {
      "number_of_replicas": 0
    }
}
'
```

API response
```js
{
  "error" : {
    "root_cause" : [
      {
        "type" : "null_pointer_exception",
        "reason" : "Cannot invoke \"String.equals(Object)\" because \"expectedValue\" is null"
      }
    ],
    "type" : "null_pointer_exception",
    "reason" : "Cannot invoke \"String.equals(Object)\" because \"expectedValue\" is null"
  },
  "status" : 500
}
```

Stack trace: 
```
  │ info [r.suppressed] [rudolf-elastic-mbp.lan] path: /_settings, params: {pretty=}
   │      java.lang.NullPointerException: Cannot invoke "String.equals(Object)" because "expectedValue" is null
   │            at org.elasticsearch.action.admin.indices.settings.put.TransportUpdateSettingsAction.checkForSystemIndexViolations(TransportUpdateSettingsAction.java:146) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.admin.indices.settings.put.TransportUpdateSettingsAction.masterOperation(TransportUpdateSettingsAction.java:93) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.admin.indices.settings.put.TransportUpdateSettingsAction.masterOperation(TransportUpdateSettingsAction.java:51) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.master.TransportMasterNodeAction$AsyncSingleAction.lambda$doStart$3(TransportMasterNodeAction.java:162) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.ActionRunnable$2.doRun(ActionRunnable.java:73) ~[elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.EsExecutors$DirectExecutorService.execute(EsExecutors.java:176) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.master.TransportMasterNodeAction$AsyncSingleAction.doStart(TransportMasterNodeAction.java:162) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.master.TransportMasterNodeAction.doExecute(TransportMasterNodeAction.java:109) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.master.TransportMasterNodeAction.doExecute(TransportMasterNodeAction.java:57) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:88) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.ActionFilter$Simple.apply(ActionFilter.java:53) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:86) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.action.filter.SecurityActionFilter.lambda$applyInternal$3(SecurityActionFilter.java:160) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.ActionListener$3.onResponse(ActionListener.java:178) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.AuthorizationService.lambda$runRequestInterceptors$15(AuthorizationService.java:381) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:128) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture$1.doRun(ListenableFuture.java:112) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.EsExecutors$DirectExecutorService.execute(EsExecutors.java:176) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.notifyListener(ListenableFuture.java:106) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.lambda$done$0(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at java.util.ArrayList.forEach(ArrayList.java:1511) [?:?]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.done(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.BaseFuture.set(BaseFuture.java:144) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.onResponse(ListenableFuture.java:127) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.StepListener.innerOnResponse(StepListener.java:62) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.NotifyOnceListener.onResponse(NotifyOnceListener.java:40) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.interceptor.BulkShardRequestInterceptor.intercept(BulkShardRequestInterceptor.java:76) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.AuthorizationService.lambda$runRequestInterceptors$14(AuthorizationService.java:376) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:128) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture$1.doRun(ListenableFuture.java:112) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.EsExecutors$DirectExecutorService.execute(EsExecutors.java:176) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.notifyListener(ListenableFuture.java:106) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.lambda$done$0(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at java.util.ArrayList.forEach(ArrayList.java:1511) [?:?]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.done(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.BaseFuture.set(BaseFuture.java:144) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.onResponse(ListenableFuture.java:127) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.StepListener.innerOnResponse(StepListener.java:62) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.NotifyOnceListener.onResponse(NotifyOnceListener.java:40) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.interceptor.FieldAndDocumentLevelSecurityRequestInterceptor.intercept(FieldAndDocumentLevelSecurityRequestInterceptor.java:65) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.interceptor.UpdateRequestInterceptor.intercept(UpdateRequestInterceptor.java:23) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.xpack.security.authz.AuthorizationService.lambda$runRequestInterceptors$14(AuthorizationService.java:376) [x-pack-security-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.action.ActionListener$1.onResponse(ActionListener.java:128) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture$1.doRun(ListenableFuture.java:112) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.EsExecutors$DirectExecutorService.execute(EsExecutors.java:176) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.notifyListener(ListenableFuture.java:106) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.lambda$done$0(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at java.util.ArrayList.forEach(ArrayList.java:1511) [?:?]
   │            at org.elasticsearch.common.util.concurrent.ListenableFuture.done(ListenableFuture.java:98) [elasticsearch-8.0.0-SNAPSHOT.jar:8.0.0-SNAPSHOT]
   │            at org.el
```

Related: https://github.com/elastic/elasticsearch/pull/65604

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PUT /_settings causes NullPointerException if a system index is present #67147

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PUT /_settings causes NullPointerException if a system index is present #67147

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